WINDS AND WEATHER 179 



about us is warm or cold, wet or dry. So we see that 

 amounts of heat and amounts of water are the chief de- 

 terminers of the nature of the weather, the atmosphere 

 being the substance in which these determiners operate. 

 Evidently, then, the study of weather is a study of local changes 

 in the state of the atmosphere as to heat and as to water. 



How are such changes brought about ? In the first place, 

 there is the warming of the atmosphere when the sun 

 shines on it, and its cooling off (refrigeration) when the sun 

 is not shining on it. This is a process which always op- 

 erates with reference to the atmosphere as a whole. But 

 evidently it seems frequently not to operate with refer- 

 ence to local conditions, for the sun often shines brightly 

 during our coldest days. So we must look further. 



Already you have noted that unequal heating of the 

 atmosphere causes atmospheric movements (convection 

 currents). You have also noted that heat received from 

 the sun may be chiefly absorbed by ice and snow, and 

 so have little effect on the atmosphere. These facts help 

 us to understand why the air may be cold even in bright 

 sunshine. 



Evidently, then, the state of the weather is determined by 

 atmospheric movements and by local conditions, more than 

 by the direct action of the sun, since these evidently can 

 overcome the direct effects of the sun. This is even more 

 evident when we consider weather as to its wet and dry 

 qualities, for the distribution of water by atmospheric 

 movements is even more familiar to us than the distribu- 

 tion of heat. We have visible evidence of water move- 

 ment in air whenever we see clouds moving across the sky. 

 Heat movement in air, though not visible, is just as real 

 as water movement, and when cold or hot winds strike 



